This invention relates to the shielding of articles in a space environment, and, more particularly, to shielding that provides thermal and electrical control while being transparent to radio-frequency energy.
A structure in space is subjected to severe conditions of heating and cooling, buildup of electrical charges, external radiation and particle attack, and passive intermodulation (PIM). These factors are all related to surface properties of the structure. The surface properties of materials that make good structures may not be optimal in regard to these other considerations. Consequently, it is known to provide some of the external surfaces of the structures with protective structures, generically termed "sunshields", that protect them from external attack and also provide positive effects including the dissipation of static charge and passive thermal control. The sunshield also must be readily affixed to the structure to be protected, yet durable. In the case of some spacecraft, such as communications satellites, the structure must remain stable and resist degradation for a period of years.
A radio frequency or radar antenna structure of a spacecraft is subject to all of these considerations. In addition, any protective structure for the antenna must be transparent to the transmission of radio frequency (RF) energy to and from the antenna. (As used herein, "antenna" includes both the transmitting/receiving element and the related structure, such as its feedhorns.) This additional requirement of RF transparency imposes a significant constraint on the sunshield, because, to some extent, the ability to prevent the buildup of electrical static charge and RF transparency are apparently incompatible. Ideally, the sunshield would be electrically conductive to bleed static charges, but a dielectric to be RF transparent.
Several approaches are known in an attempt to satisfy the shielding requirements for spacecraft antennas. In one, a polyimide film has a thin layer of germanium on one side and a thin layer of gridded vacuum-deposited aluminum on the other. This material is RF transparent, but it does not meet electrostatic discharge requirements and is a potential source for passive intermodulation problems because of the presence of the aluminum metal. In another approach, the Spar sunshield material is formed of a thick sheet of polyimide film with a white dielectric paint on one side and a black dielectric paint on the other side. Neither paint dissipates static charges, so the white-paint side is typically overcoated with a thin film of indium-tin-oxide (ITO) in an attempt to control static charges. This film material meets RF-transparency, thermal, and PIM requirements, but testing has shown that it is subject to the accumulation of excessive static charges. If the ITO coating is made sufficiently thick to dissipate static charges in a satisfactory manner, it tends to block the transmission of RF signals.
There is a need for an improved sunshield material for spacecraft RF antennas and other structures that must meet the various requirements just discussed. The present invention fulfills this need, and further provides related advantages.